Offshore storage system

ABSTRACT

An offshore storage tank having a circular cylindrical vertical shell, a continuous horizontal rigid bottom containing solid ballast joined to the shell and adapted to rest on a sea floor, said vertical shell being tall enough to project above sea level, when the bottom is on the sea floor, for a height adequate for the shell to hold, when full, a liquid to a liquid level above sea level to provide a pressure, together with the pressure of the tank weight, adequate to hold the tank in place during storm conditions at a locality where the tank is to be positioned and said vertical shell, only when supplemented by said internal liquid pressure, being able to withstand storm conditions without collapsing when the tank rests on a sea floor.

United States Patent [191 Davis et a1.

[ OFFSHORE STORAGE SYSTEM [75] Inventors: William Allen Davis, GlenEllyn;

Gerald Edward Burns, Villa Park, both of 111.

[73] Assignee: Chicago Bridge & Iron Company, Oak Brook, Ill.

22 Filed: Feb. 16, 1972 211 Appl. No.: 226,775

[451 Feb. 12,1974

Primary Examiner-Jacob Shapiro Attorney, Agent, or Firm-Merriam,Marshall, Shapiro & Klose [5 7] ABSTRACT An offshore storage tank havinga circular cylindrical vertical shell, a continuous horizontal rigidbottom containing solid ballast joined to the shell and adapted to reston a sea floor, said vertical shell being tall enough to project abovesea level, when the bottom is on the sea floor, for a height adequatefor the shell to hold, when full, a liquid to a liquid level above sealevel to provide a pressure, together with the pressure of the tankweight, adequate to hold the tank in place during storm conditions at alocality where the tank is to be positioned and said vertical shell,only when supplemented by said internal liquid pressure, being able towithstand storm conditions without collapsing when the tank rests on asea floor.

10 Claims, 4 Drawing Figures PATENTEU FEB 1 2 I974 SHEET 2 or 3 OFFSHORESTORAGE SYSTEM This invention relates to tanks for storing liquids. Moreparticularly, this invention is concerned with an offshore storage tankwhich can be floated to a site and submerged to rest on the sea floorand which can be used for storing liquid materials including liquidhydrocarbons which have a specific gravity less than the specificgravity of fresh water and sea water, as well as the storage of heavierliquids.

Large amounts of oil are obtained from offshore wells, many of which aremany miles from the nearest land. While the oil from such wells can betransported by pipeline to shore for storage, it is generally consideredmore suitable and economical for the oil to be stored close to theproducing well until the amount accumulated is adequate for a tanker toremove the oil and transport it to a suitable destination.

Many types of structures have been proposed, and a considerable numberhave actually been developed and used, for the storage of liquidhydrocarbons, such as oil, offshore. Many of the offshore storage tanks,however, are expensive to construct and to transport to an offshore sitefor submergence to the sea floor. Also, after submergence the offshoretank must be sufficiently strong to withstand the maximum stormconditions expected while the tank is in use at the site. In general,such tanks must be calculated to have adequate structural integrity towithstand what is generally referred to as a 100-year storm condition atthe site where the tank is positioned on the sea floor. In addition tomeeting such requirements for safety purposes,

' an offshore tank should be capable of holding and storing the oil atthe lowest possible cost per gallon or barrel consistent with safety andgood ecological practice.

There is provided by the subject invention an offshore storage tankwhich can be built essentially completely onshore, or in a suitable areaoffshore, and then tow-floated to a site offshore for submergence to thesea floor. The offshore tank provided by the invention can provide alarge storage capacity at a comparatively low cost per unit of storagevolume because of its relative simplicity of design and construction.The tank has a circular cylindrical vertical shell which can be made ofany suitable material but advisably is made of metal plate. The shellhowever can also be made of concrete. The tank has a continuoushorizontal rigid bottom containing solid ballast, joined to the shell.The bottom is essentially flat and is adapted to rest on top of a seafloor at a site where the tank is to be submerged for use. The verticalshell is made sufficiently tall or high enough so as to extend from thesea floor to a height above sea level adequate to store fresh water, seawater or some other liquid at a liquid level above the surrounding sealevel so as to provide a net downward force against the bottom of thetank. The combined weight of the tank plus the pressure applied by theliquid head in the tank above sea level is to be adequate to hold thetank in place during storm conditions at a locality where the tank is tobe positioned. Furthermore, the vertical shell, only when supplementedby saidd internal liquid pressure, is able to withstand storm conditionswithout buckling and collapsing when the tank rests on the sea floor.Without such a liquid level in the tank, the shell is insufficientlystrong to withstand storm conditions. It should be understood howeverthat the shell is sufficiently strong and still adequately ballasted towithstand sea pressure during nonstorm con ditions when the internalliquid level is at about sea level.

When the offshore storage tank is to be used for storing a liquidhydrocarbon, the tank is provided with means for supplying a liquidhydrocarbon thereto and for withdrawing it from the tank, and means forsupplying water to the tank and for withdrawing it therefrom. Both ofsuch means however are operable to effect the desired interchange ofliquids in the tank while simultaneously keeping the tank interior fullof liquid at least to a level above sea level so as to provide asubstantial downward pressure on the tank bottom adequate to hold thetank in place during storm conditions. The vertical shell of the tank,only when supplemented by said internal liquid pressure, is able towithstand storm conditions without buckling and collapsing when the tankrests on the sea floor.

In one embodiment, the bottom of the tank advisably includes ahorizontal metal plate which is joined to the circular verticalcylindrical metal shell. A layer of solid ballast material, such asconcrete, is located on top of the metal plate comprising part of thebottom. It is furthermore advisable for a metal plate to be located ontop of the ballast layer and to have such top metal plate joined to thecircular vertical cylindrical shell. The ballast layer is furthermoreadvisably reinforced to provide added strength. In addition, tying meansadvisably join the ballast layer to the metal plates above and below theballast layer, as well as to the circular vertical cylindrical shell.

Because the tank of the subject invention is intended to be made oflarge size, such as of at least l00 feet in diameter, the vertical metalshell is advisably made of a series of horizontal metal courses or ringsfabricated from metal plate. The thickness of each ring may be ofuniform thickness except for plates used around openings or the likewhere heavier plates are sometimes desirable for added strength.However, the rings of the shell which are subject to wave action areadvisably made thicker and thereby stronger than the other rings so thatthe tank can more readily resist the forces applied during storms. Byusing thinner metal plates for the lower rings, less expensive materialand fabrication costs are entailed.

To aid in preventing excessive bending or ovaling of a circularcylindrical vertical metal shell, horizontally positioned spaced-apartstiffener rings can be joined to the internal surface of the shell. Asdistinguished from ovaling, which does not affect the integrity of thetank, the liquid head maintained in the tank is necessary to keep thetank shell from buckling and collapsing even though it permits ovaling.Ovaling is, of course, less of a problem with a concrete shell since itis far less flexible than a metal shell.

A skirt can be vertically positioned to extend downwardly from the lowerpart of the tank into the sea floor to prevent sea bed material fromeroding from beneath the tank bottom. The skirt more specifically ispositioned to project downwardly from the lower outer edge of thevertical shell. The skirt is forced into the sea floor when the tank issubmerged to rest on the sea floor.

The tank can be left open at the top or it can'be covered with a fixedposition roof of a domed or conical shape. In addition, a substantiallyflat fixed position roof can be employed. Such fixed position roof canbe mounted on the upper part of the circular cylindrical vertical shell.However, a floating roof can be employed inside of the verticalcylindrical shell with or without a fixed outer roof. However, since thetank is to be kept full of a liquid above the level of the surroundingsea, the vertical displacement of the floating roof advisably is limitedso as not to go significantly below sea level when the tank is inoperation. To provide for free vertical displacement of a floating roofon the liquid in the tank when the vertical shell has interiorspaced-apart stiffening rings, it is advisable to position a cylindricalroof guiding metal shell over the stiffeners in the upper portion of thetank vertical shell.

The invention will be described further in conjunction with the attacheddrawings in which:

FIG. 1 is a vertical cross-sectional view of an offshore storage tank,provided according to the invention, havng an open top and a floatingroof;

FIG. 2 is a enlarged view of the bottom structure of the offshorestorage tank of FIG. 1;

FIG. 3 is another embodiment of an offshore storage tank provided by theinvention shown in vertical sectional view resting on a sea floor andhaving a stationary roof; and

FIG. 4 is a vertical sectional view of'a tank such as shown in FIG. 1 inwhich the vertical metal shell is thicker at the area of wave actionthan the areas below and above.

So far as is practical, the same parts or elements which are identicaland appear in the different figures of the drawings will be identifiedby the same numbers.

With reference to FIGS. 1 and 2, the offshore storage tank has avertical cylindrical metal shell or wall ll made of a plurality ofhorizontal courses or rings of metal plate. Each ring is circular inhorizontal section and can be of any suitable height but generally willbe from about six to 10 feet tall. Any number of such rings can beemployed to make the vertical cylindrical shell as high as desired, butat least high enough to extend substantially above sea level at a sitewhere the tank is to besubmerged. The bottom of tank 10 has a lowerhorizontal metal plate 12 and an upper horizontal metal plate 13, bothof which extend over the area circumscribed by vertical shell 11.Concrete is positioned between bottom plate 12 and upper plate 13 andconstitutes ballast material. Sufficient ballast is employed to firmlyanchor the tank on the sea floor when it is submerged and contains aliquid to a level above sea level adequate to prevent the tank shellfrom collapsing during storm conditions.

A plurality of metal stiffener rings 15 can be positioned on the insidesurface of vertical shell 11 in spaced-apart horizontal relationshipwith respect to one another. The stiffener rings however can be placedon the outside of the shell. Skirt 16, made of metal plate, ispositioned around the lower bottom portion of the vertical metal shell.It extends below bottom plate 12 for a significant distance so that itcan penetrate into the sea floor as the tank is submerged to restthereon. Grout I7 is placed between skirt l6 and vertical shell 11 tohold the skirt firmly in place before the tank is submerged.

Floating roof 19 is displaceable vertically with rise and fall of liquidin the tank. Vertical metal plate ring 18 is located in the top innerportion of vertical shell 11 to provide a cylindrical wall having asmooth surface against which circular floating roof 19 may rub withoutobstruction during vertical displacement.

Standpipe 20 communicates at its lower end 21 with the internal bottomspace of tank 10. The upper outlet end 22 of standpipe 20 is located atthe maximum water level which tank 10 is designed to hold to provideextra downward pressure to seat the tank firmly on the sea floor so thatit can safely withstand storm action. Conduit 23 is used to feed oilinto storage tank 10 and to remove oil therefrom. Conduit 23 is locatedat the same height as the water level when the tank is full of waterand, therefore, it is at the same height as the overflow end 22 ofstandpipe 20. Conduit 24 communicates with the lower internal portion oftank 10 and with pump 25 for pumping water into the tank when oil is tobe removed therefrom.

FIG. 2 illustrates the construction of the bottom of the tank in moredetail than is shown in FIG. 1. A plurality of spaced-apart metalreinforcing members 31 and 33 are positioned horizontally to form acriss-cross pattern above metal plate 12 and are held there by shearconnectors 32, which are also attached to plate 12, until the concretesolidifies. Similarly, reinforcing member 34 are positioned spacedbeneath metal plate 13 and reinforcing members 35 are positioned spacedapart from one another lateral to reinforcing members 34. Shearconnectors 36 maintain the reinforcing bars in position relative to oneanother and plate 13. The perimeter of concrete ballast 14 hasreinforcing bars 37 and 38 crisscrossing one another and held in fixedspaced-away position from vertical shell 11 by shear connectors 39,which are joined to the shell 11, until the concrete hardens.

The offshore storage tank 10 shown in FIGS. 1 and 2 can be built onshorein a graving dock and thereafter floated to a site for submergence.Submergence of the tank can be achieved by pumping sea water into thetank. The tank may tilt to 10 or 15 in the early stages of descent butwill become stable in the upright position as it submerges to deeperdepths.

A tank according to the embodiment of FIGS. 1 and 2 should provide anadequate downward net pressure by the weight of the material used in thetank plus concrete ballast 14 to hold the tank securely against the seafloor when the tank has at least no more liquid therein than is equal tothe pressure exerted by a water content in the tank up to sea level.However, to minimize the amount of ballast which need be employed, it isadvisable to always have a liquid content in the tank at a levelsubstantially above sea level so that additional downward pressure isprovided to stabilize the tank. The tank of FIGS. 1 and 2 is so designedas to have such a liquid head above sea level to provide the addeddownward pressure. Such liquid head is however required for the tankshell to withstand storm conditions without collapsing. Therefore, suchliquid head is advisably always maintained in the tank once it issubmerged.

The operation of the tank of FIGS. 1 and 2 in storing oil is notcomplicated. When the tank 10 is positioned in place on a sea floor, thewater level therein is raised to the level of the overflow end 22 ofstandpipe 20. To fill the tank with oil, oil is pumped into the tankthrough conduit 23. Because the oil naturally has a specific gravityless than sea water, floating roof 19 will rise and, at the same time,the interface between the oil and the water will move downwardly. Thepressure head thereby developed inside of tank will cause water to flowfrom the tank through standpipe and out outlet end 22. As pumping of oilinto the tank continues, the oil-water interface will continue droppingin tank 10 and is permitted to drop until it reaches the lower end 21 ofthe standpipe. At that point the tank is considered full of oil so thepumping of oil is terminated. Floating roof 19 is displaced upwardlywhen the tank is filled with oil until it is essentially located at thetop edge of vertical cylindrical shell 11. When the tank is full of oil,floating roof 19 will be in its most upward position shown in phantom as19A. When the tank is full of oil, the net pressure of the oil on thetank bottom will be essentially identical to the net pressure obtainedwhen the tank is full of water to the level of the outlet 22 ofstandpipe 20 so that as a result there is a substantially uniformpressure exerted on the tank bottom whether the liquid in the tank isoil, water or two separate layers thereof. Such uniform pressure iscalculated to keep the tank shell from collapsing during stormconditions.

To remove oil from the tank of FIG. 1, a valve (not shown) in conduit 23can be opened and, as a result, oil will flow out therefrom. Water canbe pumped in by conduit 24 by means of pump 25 to force the oil level upsufficiently high so that oil will flow out conduit 23 by gravity.

With reference to FIG. 3, the tank 40 shown therein is intended foroffshore use for the storage of oil. Tank 40 has a vertical circularcylindrical metal shell 41 which is attached or connected at its lowerbottom portion to concrete ballast slab 42 having no metal top orbottom, although either or both can be used if desired. Concrete ballastslab 42 has a peripheral edge 50 which extends beyond shell 41 torestrict the tank against tipping during storm conditions. Skirt 51extends from the bottom of ballast slab 42 into the sea floor to keepsea bed material from being eroded from beneath ballast slab 42.

Stiffener rings 43 are positioned horizontally in spaced-apartrelationship inside of vertical shell 41 to reinforce it against bendingthrough the pressure of the sea and forces developed through wave and ofthe sea and forces developed through wave and storm action. If desired,the stiffener rings can be positioned on the outside of the shell. Metalsemielliptically domed roof 44 is supported by vertical shell 41. Vent45 in domed roof 44 permits flow of air in both directions to preventdevelopment of an internal pressure different from atmospheric pressure.

The offshore storage tank of FIG. 3 can be constructed onshore in agraving dock and then floated to a suitable site for submergence. It canbe submerged by pumping water inside of the tank with care being takenin the tank design so that the weight distribution prevents uncontrolledtilting and capsizing. Once the tank is partially submerged, and thecenter of gravity is below the center of buoyancy, it will be in astable condition and no further means need be employed to stabilize thetank during further submergence.

The tank of FIG. 3 is provided with sufficient weight by ballast 42,vertical shell 41 and roof 44 to provide an adequate downward pressurewhen full of water to sea level sufficient to maintain the tank in astable position on a sea floor site. However, to provide additionaldownward force for added stability, tank 40 is designed to normally holdwater up to the line WP, and to hold oil up to the line OP, shown inFIG. 3. The oil level line OP and the water level line WP are sopositioned as to provide essentially the same pressure on the tankbottom when the tank is full with oil or water to the designated level.The oil level line OP is located above the water level line WP becauseoil has a specific gravity (about 0.85) lower than water and therefore alarger volume of oil must be stored in the tank to obtain a pressureequal to the pressure exerted by a volume of water in the tank having alevel at the water level line WP. Both the oil level line OP and thewater level line WP are located above sea level so that a suitablepressure head is maintained under most circumstances to provide thedesired downward pressure on the tank bottom.

Inlet-outlet conduit 46 is located at water level line WP. Furthermore,outlet 48 of standpipe 47 is also positioned at the water level line WP.The lower end 49 of standpipe 47 communicates with the lower internalspace of the tank. Pump 52 is used to pump water through conduit 53 intothe lower space of tank 40.

When tank 40 is not employed for storing oil, it is maintained full ofwater up to the water level line WP. When it is to be used for oilstorage, oil is pumped in through conduit 46. As the volume of oilsupplied to tank 40 increases, the interface between the oil layer ontop of the water continually descends and water is forced throughstandpipe 47 and out outlet 48. Once the oil-water interface descends tothe lower end 49 of the standpipe, pumping of oil to the tank isterminated. At such point, the level of oil in the tank will be at theoil level line OP.

When the tank 40 is full of oil, oil may be removed through conduit 46.When oil is removed through conduit 46, water is pumped into the tank bypump 52 through conduit 53. Pumping of water is continued until thewater level in the tank reaches the line WP so that there is an addeddownward pressure against the tank bottom produced by the height of thewater in the tank above the external sea level.

Operation and use of the tank, the means for supplying oil to the tank,and the means for withdrawing water from the tank, are intended tomaintain a total liquid content in the tank which exerts a hydrostaticpressure on the tank bottom per unit of area above the external seapressure. The result of this is that the weight of the tank, includingthe ballast, always applies a downward pressure which holds the tankstable under normal sea conditions. By maintaining the internal water oroil level above sea level, the downward pressure on the bottom providesstability adequate for the tank shell wall to withstand design stormconditions.

EXAMPLE 1 An offshore storage tank for oil is constructed for placing ona sea floor at a site having a mean sealevel of feet above the seafloor. The tank is circular in horizontal cross-section and has avertical cylindrical metal shell 176 feet high and is of the generalshape shown in FIG. 1. A six-foot thick horizontal concrete bottom slabhaving a metal cover on each side provides ballast. A skirt 16 isprepositioned to penetrate into the sea floor for five feet. The tankhas a floating roof which is vertically displaceable within acylindrical ring 18 having a 208-foot diameter and a 32-foot height. Thetank is designed to have a maximum internal water level of feet from thebottom of the sea and a maximum internal oil level of 174 feet from thesea bottom.

The vertical cylindrical shell 11 is constructed of 22 horizontal ringseight feet high as shown in FIG. 4. Each ring is made of steel plate.The thickness of the rings starting at the bottom of the shell andprogressing upwardly is shown in FIG. 4 as follows: four rings inchthick (A,A four rings inch thick (B -B three rings 1 inch thick (C -Cone ring 1 inches thick (D one ring 1% inches thick (15,); one ring 1%inches thick (F two rings 1% inches thick (G G one ring 1 inches thick(H one ring 1% inches thick one ring 1 inch thick (J one ring inch thick(K and two rings )4; inch thick (L,L The vertical shell 11 of the tankis therefore thicker at the area effected by wave action than the areasbelow and above.

When full of water to the intended height, the tank will have a positivepressure of 2000 lbs/sq. ft. and the same pressure is intended to beapplied when the tank is full of oil. The capacity of the tank isslightly over 1,000,000 barrels of oil.

When the described tank is placed on a sea floor and filled with liquid,no piles are needed to maintain it stable even with 40 foot waves, suchas might develop during a storm condition. The tank has sufficient deadweight to provide stability for storms involving maximum wave heights of15 feet. Therefore, the tank could fill with water during tanker loadingof oil until the internal water level is at the same level as theexternal sea level, after which additional water can be added to thetank to the maximum water level line 19 feet above sea level. A 48-foothead of oil (specific gravity 0.85) in the tank gives a positivepressure equivalent to a head of 19 feet of water.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom, as modifications will be obvious to those skilled in the art.

What is claimed is:

1. In combination, an offshore storage tank supported by a sea floorcomprising:

a circular cylindrical vertical metal shell;

a continuous horizontal rigid bottom containing solid ballast, joined tothe shell and resting on the sea floor;

said vertical metal shell extending from the rigid bottom upwardly toabove sea level for a liquid to a height adequate for the shell to holdliquid to a liquid level above the level of the sea surrounding the tankto provide a pressure together with the pressure of the tank weight,which is adequate to hold the tank in place on the sea floor duringstorm conditions;

said vertical shell only when supplemented by said internal liquidpressure being able to withstand storm conditions without buckling andcollapsing with the tank resting on the sea floor; and

liquid in the shell to a level above the surrounding sea which isadequate to hold the tank in place on the sea floor during stormconditions without buckling and collapsing.

2. A combination according to claim 1 in which the tank is covered by afixed-position roof supported above sea level by the vertical metalshell.

3. In combination, an offshore storage tank supported by a sea floorcomprising:

a circular cylindrical vertical metal shell;

a continuous horizontal rigid bottom containing solid ballast, joined tothe shell and resting on the sea floor;

said vertical shell extending from the rigid bottom upwardly to abovesea level for a height adequate for the shell to hold a liquid to aliquid level above the level of the sea surrounding the tank to providea pressure together with the pressure of the tank weight, which isadequate to hold the tank in place on the sea floor during stormconditions;

said vertical shell only when supplemented by said internal liquidpressure being able to withstand storm conditions without buckling andcollapsing with the tank resting on the sea floor;

means for supplying a liquid hydrocarbon to, and for withdrawing itfrom, the tank, and means for supplying water to the tank andwithdrawing it therefrom;

said means for supplying a liquid hydrocarbon to, and withdrawing itfrom, the tank and said means for supplying water to, and withdrawingwater from, the tank being operable to maintain a total liquid contentin the tank which exerts a pressure on the tank bottom per unit areagreater than the external sea pressure; and

liquid in the shell to a level above the surrounding sea which isadequate to hold the tank in place on the sea floor during stormconditions without buckling and collapsing.

4. A combination according to claim 3 in which the bottom comprises ametal plate on top of which is a substantially uniform layer ofconcrete.

5. A combination according to claim 4 in which a metal plate is locatedon top of the concrete layer and is joined to the vertical shell.

6. A combination according to claim 3 in which a metal skirt extendsvertically downwardly from the tank bottom.

7. A combination according to claim 3 in which the meansfor withdrawingwater from the tank includes a conduit inlet communicating with thebottom interior space of the tank and an outlet located at the height ofthe maximum water level which the tank is calculated to hold.

8. A combination according to claim 3 in which the means for withdrawingliquid hydrocarbon from the tank includes a conduit having an inlet inthe tank at the height of the maximum water level which the tank iscalculated to hold.

9. An offshore storage tank comprising:

a circular cylindrical vertical metal shell made of horizontal metalrings with the rings of the shell subject to wave action thicker thanthe other rings in the vertical metal shell;

a continuous horizontal rigid bottom comprising solid ballast, joined tothe shell and adapted to rest on a sea floor;

said vertical shell being tall enough to project above sea level, whenthe bottom is on the sea floor, for a height adequate for the shell tohold, when full, a liquid to a liquid level above sea level to provide apressure together with the pressure of the tank weight, adequate to holdthe tank in place during storm conditions at a locality where the tankis to be positioned; and

said vertical shell only when supplemented by said internal liquidpressure being able to withstand storm conditions without collapsingwhen the tank rests on the sea floor.

10. An offshore storage tank comprising:

a circular cylindrical vertical metal shell having a plurality ofhorizontal spaced-apart stiffener rings joined to the internal surfacethereof;

said vertical metal shell having a cylindrical metal roof-guiding shellinside the upper portion thereof ballast, joined to the shell andadapted to rest on a sea floor;

, said vertical shell being tall enough to project above sea level, whenthe bottom is on the sea floor, for a height adequate for the shell tohold, when full,

A a liquid to a liquid level above sea level to provide a pressuretogether with the pressure of the tank weight, adequate to hold the tankin place during storm conditions at a locality where the tank is to bepositioned; and said vertical shell only when supplemented by saidinternal liquid pressure being able to withstand storm conditionswithout collapsing when the tank rests on the sea floor.

4 2 UNTTEE STATES PATENT OFFICE- CERTIFICATE 0F CORREC O .Patent No.3,791,152 Dated February 12, 1974 Inventor) William Allen Davis andGerald Edward Burns .It is certified that error appears in theabove-identified patent .and that said Letters Patent are herebycorrected as shown below:

Column 1, line 63, change "saidd" to said column 7, line 11, change '1inches" to -l-3/8 inches-; line 47,

delete "liquid to a"; column 8, line 55, change "comprising" to--containing-.

Signedj'a-nd sealed this 3rd day of September 1974.

csEAL Attestt FMcCOY M. GIBSON, JR. a c. .MARSHALL EANN AttestingOfficer Commissioner of Patents

1. In combination, an offshore storage tank supported by a sea floorcomprising: a circular cylindrical vertical metal shell; a continuoushorizontal rigid bottom containing solid ballast, joined to the shelland resting on the sea floor; said vertical metal shell extending fromthe rigid bottom upwardly to above sea level for a height adequate forthe shell to hold liquid to a liquid level above the level of the seasurrounding the tank to provide a pressure together with the pressure ofthe tank weight, which is adequate to hold the tank in place on the seafloor during storm conditions; said vertical shell only whensupplemented by said internal liquid pressure being able to withstandstorm conditions without buckling and collapsing with the tank restingon the sea floor; and liquid in the shell to a level above thesurrounding sea which is adequate to hold the tank in place on the seafloor during storm conditions without buckling and collapsing.
 2. Acombination according to claim 1 in which the tank is covered by afixed-position roof supported above sea level by the vertical metalshell.
 3. In combination, an offshore storage tank supported by a seafloor comprising: a circular cylindrical vertical metal shell; acontinuous horizontal rigid bottom containing solid ballast, joined tothe shell and resting on the sea floor; said vertical shell extendingfrom the rigid bottom upwardly to above sea level for a height adequatefor the shell to hold a liquid to a liquid level above the level of thesea surrounding the tank to provide a pressure together with thepressure of the tank weight, which is adequate to hold the tank in placeon the sea floor during storm conditions; said vertical shell only whensupplemented by said internal liquid pressure being able to withstandstorm conditions without buckling and collapsing with the tank restingon the sea floor; means for supplying a liquid hydrocarbon to, and forwithdrawing it from, the tank, and means for supplying water to the tankand withdrawing it therefrom; said means for supplying a liquidhydrocarbon to, and withdrawing it from, the tank and said means forsupplying water to, and withdrawing water from, the tank being operableto maintain a total liquid content in the tank which exerts a pressureon the tank bottom per unit area greater than the external sea pressure;and liquid in the shell to a level above the surrounding sea which isadequate to hold the tank in place on the sea floor during stormconditions without buckling and collapsing.
 4. A combination accordingto claim 3 in which the bottom comprises a metal plate on top of whichis a substantially uniform layer of concrete.
 5. A combination accordingto claim 4 in which a metal plate is located on top of the concretelayer and is joined to the vertical shell.
 6. A combination according toclaim 3 in which a metal skirt extends vertically downwardly from thetank bottom.
 7. A combination according to claim 3 in which the meansfor withdrawing water from the tank includes a conduit inletcommunicating with the bottom interior space of the tank and an outletlocated at the height of the maximum water level wHich the tank iscalculated to hold.
 8. A combination according to claim 3 in which themeans for withdrawing liquid hydrocarbon from the tank includes aconduit having an inlet in the tank at the height of the maximum waterlevel which the tank is calculated to hold.
 9. An offshore storage tankcomprising: a circular cylindrical vertical metal shell made ofhorizontal metal rings with the rings of the shell subject to waveaction thicker than the other rings in the vertical metal shell; acontinuous horizontal rigid bottom comprising solid ballast, joined tothe shell and adapted to rest on a sea floor; said vertical shell beingtall enough to project above sea level, when the bottom is on the seafloor, for a height adequate for the shell to hold, when full, a liquidto a liquid level above sea level to provide a pressure together withthe pressure of the tank weight, adequate to hold the tank in placeduring storm conditions at a locality where the tank is to bepositioned; and said vertical shell only when supplemented by saidinternal liquid pressure being able to withstand storm conditionswithout collapsing when the tank rests on the sea floor.
 10. An offshorestorage tank comprising: a circular cylindrical vertical metal shellhaving a plurality of horizontal spaced-apart stiffener rings joined tothe internal surface thereof; said vertical metal shell having acylindrical metal roof-guiding shell inside the upper portion thereofover the stiffeners adjacent thereto and the cylindrical metalroof-guiding shell has a height from maximum water level to maximum oillevel in the tank; a floating roof having a periphery in close proximityto the cylindrical metal roof-guiding shell internal surface; acontinuous horizontal rigid bottom containing solid ballast, joined tothe shell and adapted to rest on a sea floor; said vertical shell beingtall enough to project above sea level, when the bottom is on the seafloor, for a height adequate for the shell to hold, when full, a liquidto a liquid level above sea level to provide a pressure together withthe pressure of the tank weight, adequate to hold the tank in placeduring storm conditions at a locality where the tank is to bepositioned; and said vertical shell only when supplemented by saidinternal liquid pressure being able to withstand storm conditionswithout collapsing when the tank rests on the sea floor.